The Mountain That Japan Hid From The World

Inside the Shikotsu-Toya National Park, in the island of Hokkaidō, not far from the active stratovolcano, Mount Usu, there is a 400-meter tall volcanic peak called Shōwa-shinzan. Shōwa-shinzan is Japan’s youngest mountain. It appeared on 28 December 1943 out of a wheat field accompanied by strong tremors and hot lava. As the molten magma broke through the surface, it uplifted the field and over the following two years the lava dome continued to rise until it reached a height of 398 meters.

Shōwa-shinzan erupted when Japan was fighting the Allies in the Second World War. The appearance of a volcano at a time when the entire country was in distress was taken as a bad omen by the superstitious folks. The authorities tried to hush it up and requested the locals to keep the mountain a secret. But Masao Mimatsu, a postmaster living nearby began observing and recording the volcano’s progress. Because of Japan’s war effort, basic scientific materials were unavailable and Mimatsu had to improvise. The notes he took and the sketches he made of Showa-Shinzan are the only records available to geologists of this mountain’s formation.

Masao Mimatsu strung several fishing lines horizontally across two vertical beams at his post office. He observed the growth of the volcano through these strings—they acting as guide lines—and drew the profile of Showa-Shinzan at different times as the dome grew. When he presented his data and sketches to the World Volcano Conference in Oslo in 1948, his work was praised by professional volcanologists. His papers were referred to as the “Mimatsu Diagram” and for them he received the First Hokkaido Cultural Award.

Mimatsu also bought the entire land where the volcano stood using all of his savings so that he could study the volcano more thoroughly. Some say this happened when the volcano was still growing. Others say he bought the land after the volcano stopped erupting. Whichever is true, Mimatsu became the owner of the volcano and to this day the volcano is on private property despite the Japanese government declaring it a natural monument of Japan—a rarity anywhere in the world.

Masao Mimatsu is honored by the Mimatsu Masao Memorial Hall near the site of Shōwa-shinzan and a bronze statue of him looking through a surveying equipment.